Method for the dissolution and purification of tantalum pentoxide

ABSTRACT

A method is disclosed for the dissolution and purification of tantalum pentoxide. The impure tantalum pentoxide is reacted with a potassium-containing compound to form potassium tantalate. The potassium tantalate is optionally slurried with sulfuric acid and dissolved in an HF medium. The solution is suitable for purification by conventional ion exchange or solvent extraction methods. A potassium fluorotantalate precipitate may also be formed by adding KCl to the solution. The fluorotantalate precipitate may be further processed into a pure tantalum pentoxide by suspending the precipitate in an aqueous solution optionally containing a chelating agent and adding ammonium hydroxide to form ammonium tantalum oxide which can then be converted to tantalum pentoxide by calcining at high temperature.

TECHNICAL FIELD

This invention relates to methods for the processing and purification oftantalum-containing ores and compounds. In particular, this inventionrelates to methods of producing high purity tantalum compounds fromstandard technical grade tantalum pentoxide.

BACKGROUND ART

High purity tantalum metal and tantalum pentoxide have becomeincreasingly important to the electronics industry in the preparation ofadvanced electronic materials used in the manufacture of devices such assurface acoustic wave filters, pyroelectric infrared sensors andoptoelectronic devices. High purity tantalum pentoxide is also requiredfor the preparation of tantalate x-ray phosphors for x-ray intensifierscreens. The purity of tantalum metal and tantalum pentoxide used in themanufacture of such products should be greater than 99.99% andpreferably approach 99.9999%.

Three major methods exist for purifying tantalum-containing materials:distillation (or chlorination), liquid-liquid solvent extraction and ionexchange.

In the distillation method, a mixture of an impure tantalum-containingmaterial and carbon is reacted with chlorine gas at about 600° to 800°C. Although distillation is a well established commercial process forthe purification of tantalum-containing materials, it has difficultyseparating tantalum and niobium because of the small difference in theboiling points of tantalum chloride (239° C.) and niobium chloride (249°C.). Since niobium is a ubiquitous impurity in tantalum-containingmaterials, solvent extraction and ion exchange methods are preferredover distillation when high purity tantalum and tantalum pentoxide aredesired.

In solvent extraction methods, an impure tantalum-containing material isdissolved in hydrofluoric acid (HF) media and brought into contact withan organic solvent. Tantalum values are selectively extracted into theorganic phase, back-extracted into aqueous solution, and precipitated astantalum hydroxide by hydrolyzing with aqueous ammonia. The precipitatedhydroxide can be converted to tantalum pentoxide by calcining in air.However, multiple extraction/back-extraction cycles may be necessary toproduce the desired high purity. Examples of liquid-liquid solventextraction methods are disclosed in U.S. Pat. Nos. 3,117,833, 3,712,939and 4,673,554.

In ion exchange methods, an impure tantalum-containing material isdissolved in HF media and then passed through an anion exchange columnwhich selectively retains tantalum metal values. The impurities remainin solution and are discharged in the effluent. The retained tantalumvalues are subsequently eluted from the column and then precipitated astantalum hydroxide as above. An example of such a method is disclosed inU.S. Pat. No. 4,446,115. However, because of the fairly small exchangecapacity of the anion exchange resin, this method is expensive to applyon a commercial scale.

While the prior art focuses the purification of tantalum-containing oreand scrap, there exists a large inventory of technical grade tantalumpentoxide which could provide a relatively inexpensive raw material forthe production of high purity tantalum and tantalum oxide. However,standard technical grade tantalum pentoxide contains a number ofdifferent impurities (e.g., Al, Si, F, Cl, Na, Cr, Fe, Co, Ni, Cu, Ti,Zr, Mo, Nb, and W) at levels between 500 to 10,000 parts per million(ppm). Such high impurity levels would be unacceptable for use in theabove electronic and phosphor applications. Thus, it is necessary topurify technical grade tantalum pentoxide before it can be used.

Unfortunately, tantalum pentoxide is extremely difficult to dissolveeven in hydrofluoric acid, especially the crystalline β-Ta₂ O₅ phase.Direct dissolution in HF requires excessively long dissolution times andmany times the amount of HF dictated by the stoichiometry of thereaction. Thus, direct dissolution of Ta₂ O₅ for use in conventionalpurification methods like solvent extraction and ion exchange iscommercially impractical.

The chemical reaction for the dissolution of Ta₂ O₅ in HF can be writtenas:

    Ta.sub.2 O.sub.5 +14HF(aq)→2H.sub.2 TaF.sub.7 (aq)+5H.sub.2 O

The reaction kinetics of the dissolution of Ta₂ O₅ in HF media have beeninvestigated by I. I. Baram, Journal of Applied Chemistry of theU.S.S.R., V. 38, 2181-88 (1965). According to the reported results, onlyabout 5.3 g of pure Ta₂ O₅ (which had been calcined at 800° C. for 3hours) could be dissolved in 1 l of 14.6M HF in 4 hours at 70° C.Because the dissolution reaction was reported to be first order withrespect to HF concentration, it can be estimated that approximately 10.8g of Ta₂ O₅ would have dissolved in 1 l of 29 M HF in 4 hours at 70° C.,a molar ratio of approximately 1200:1 HF/Ta₂ O₅. Thus, it is evidentthat under the above conditions, much more HF is required to dissolveTa₂ O₅ than would be dictated by stoichiometry. Such a large excess ofHF is not only economically undesirable but is also likely to behazardous.

Although increasing the temperature will increase the rate ofdissolution, the amount of HF required for dissolution is still manytimes the stoichiomteric amount. For example, over a period of 3 days at100° C., approximately 15 g of Ta₂ O₅ can be dissolved in 190 ml ofapproximately 25M HF, a molar ratio of about 140:1 HF/Ta₂ O₅, or about10 times the stoichiometric amount. However, using higher dissolutiontemperatures also increases energy consumption and the amount ofdangerous HF fumes.

Therefore, it would be a significant improvement in the art to have amethod of dissolving Ta₂ O₅ in an HF media to produce atantalum-containing solution which could be used directly inconventional solvent extraction and ion exchange purification methods.It would also be a significant improvement to have a method ofdissolving Ta₂ O₅ in HF media which would require much less HF thanneeded for direct dissolution of Ta₂ O₅ in HF media under the sameconditions.

SUMMARY OF THE INVENTION

It is an object of the invention to obviate the disadvantages of theprior art.

It is another object of the invention to provide an economical and safemethod for the dissolution and purification of tantalum pentoxide.

It is a further object of the invention to provide a method for thedissolution and purification of tantalum pentoxide without utilizingsolvent extraction or ion exchange techniques.

In accordance with one aspect the invention, a method is provided forthe dissolution and purification of tantalum pentoxide comprisingreacting tantalum pentoxide and a potassium-containing compound to formpotassium tantalate, dissolving the potassium tantalate in ahydrofluoric acid medium to form a solution containing tantalum valuesand impurities and separating the tantalum values from the impurities.

In accordance with another aspect of the invention, a method is providedfor the dissolution and purification of tantalum pentoxide comprisingreacting stoichiometric amounts of an impure tantalum pentoxide andpotassium carbonate to form potassium tantalate, optionally slurryingthe potassium tantalate with sulfuric acid, dissolving the potassiumtantalate in an hydrofluoric acid medium to form a solution containingtantalum values and impurities, precipitating potassium fluorotantalateby adding a water soluble potassium compound, filtering the solution toobtain a wet precipitate, suspending the wet precipitate in water,optionally adding an effective amount of a chelating agent,precipitating (NH₄)₂ Ta₂ O₆ ·H₂ O by adding ammonium hydroxide andcalcining the (NH₄)₂ Ta₂ O₆ ·H₂ O to obtain a tantalum pentoxide havinglow amounts of niobium and transition metals.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a block diagram illustrating a method of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims taken inconjunction with the above-described drawings. Examples are provided forillustration purposes and should not be construed as limiting theinvention to a particular embodiment.

In order to purify technical grade tantalum pentoxide using conventionalsolvent extraction or ion exchange techniques, it is necessary to firstdissolve the impure Ta₂ O₅ in an HF media. To avoid the difficultiesencountered with dissolving Ta₂ O₅ directly in an HF media, the impureTa₂ O₅ is first converted to potassium tantalate, KTaO₃, by reacting theTa₂ O₅ with a stoichiometric amount of a potassium-containing compound,preferably, potassium carbonate, K₂ CO₃, at a temperature between about550° C. to about 1300° C. for a time between about 3 to about 20 hours.Other sources of potassium may also be used, e.g., KCl, K₂ O, K₂ C₂ O₄,KHCO₃, etc. The preferred reaction proceeds according to the followingequation:

    Ta.sub.2 O.sub.5 →K.sub.2 CO.sub.3 →2KTaO.sub.3 +CO.sub.2 (g)↑

The preferred temperature and time for this reaction are about 750° C.to about 850° C. for about 5 hours to about 10 hours

The conversion of standard technical grade Ta₂ O₅ to KTaO₃ yields a veryefficient method of dissolving tantalum pentoxide in HF media, sinceKTaO₃ almost instantaneously dissolves in HF media. Moreover, during thepreparation of KTaO₃, the purity of technical grade Ta₂ O₅ is improvedwith respect to W, Mo, Cr and Nb. This increase in purity is exemplifiedin Table 2. Additionally, the perovskite form of KTaO₃ may be madeexclusively at 700° C. from stoichiometric amounts of K₂ CO₃ and Ta₂ O₅without forming any of the metabolic form of KTaO₃ having the pyrochlorecubic structure.

                  TABLE 2                                                         ______________________________________                                                       Impurity Concentration                                                        With Respect to Ta,                                                           ppm (unless otherwise                                                         indicated)                                                     Elements         Ta.sub.2 O.sub.5                                                                              KTaO.sub.3                                   ______________________________________                                        Potassium        <2              12%                                          Arsenic          <1              <1                                           Bismuth          <1              <1                                           Chromium         15              6                                            Cobalt           <1              <1                                           Copper           21              2                                            Iron             64              59                                           Lead             2               <1                                           Manganese        2               2                                            Molybdenum       <1              <1                                           Nickel           9               7                                            Niobium          250             218                                          Thorium          <1              <1                                           Tin              2               <1                                           Titanium         <1              <1                                           Tungsten         140             108                                          Vanadium         <1              <1                                           Zinc             4               2                                            Zirconium        <1              <1                                           ______________________________________                                    

After the KTaO₃ has been formed, it is cooled, washed with water, anddried at 110° C. The KTaO₃ reacts strongly with HF and can be dissolvedalmost instantaneously at room temperature. However, for a moreefficient dissolution process, the KTaO₃ is slurried first with sulfuricacid, H₂ SO₄, before dissolution in HF.

EXAMPLE 1

10 ml of concentrated (49-51%) HF acid were added to 5 g of KTaO₃prepared from technical grade Ta₂ O₅. An immediate and vigorousexothermic reaction took place and almost all of the solid material wasdissolved. The solution was diluted to 200 ml with water andprecipitated with 7.4M ammonium hydroxide. The precipitate was analyzedby Glow Discharge Mass Spectrometry (GD-MS) which determined that only0.3% by weight potassium remained in the precipitate thereby indicatingthat more than 97% of the KTaO₃ dissolved in the HF. Thus, a tantalumpentoxide solution having 16 to 20 g Ta/l can easily be made inapproximately 1.5M (or 2% HF).

EXAMPLES 2-3

25 g of KTaO₃ prepared from technical grade Ta₂ O₅ was slurried with 25ml of concentrated H₂ SO₄ and then 25 ml of concentrated (29M) HF wasadded. A colorless paste was obtained as a result of the strongexothermic reaction. The paste completely dissolved in dilute (approx.1.5M) HF to yield a clear solution containing approximately 17 g/l Ta.

By comparison, under similar conditions, 25 g of technical grade Ta₂ O₅was slurried with 25 ml of concentrated H₂ SO₄ and then 25 ml ofconcentrated HF was added. No visible reaction was observed between thesolid and the acid. The solid was separated from the HF, washed withwater, dried and weighed. Only 1.2 g of the initial 25 g of Ta₂ O₅ wasfound to have dissolved in the HF media.

Thus, under similar conditions, 20.6 g of technical grade Ta₂ O₅(equivalent to 25 g KTaO₃) can be easily dissolved in HF media by firstconverting the Ta₂ O₅ to KTaO₃ whereas only 1.2 g of Ta₂ O₅ can bedissolved by direct dissolution.

EXAMPLE 4

125 g of KTaO₃ made from technical grade Ta₂ O₅ was dissolved in lessthan 10 minutes in 5.5 l of a dilute HF/H₂ SO₄ solution containing 225ml of concentrated HF and 200 ml of concentrated H₂ SO₄. Thus, it isrelatively easy to produce a solution containing 15 g/l Ta using adilute HF/H₂ SO₄ solution (1% HF and 1.3N H₂ SO₄).

Other alkali tantalates do not exhibit the same advantageous dissolutionproperties as KTaO₃. For example, tests comparing the dissolutionproperties of KTaO₃ with NaTaO₃ and LiTaO₃ found that under similarconditions only 25% of the NaTaO₃ and LiTaO₃ would dissolve in an HFmedium as compared to almost 100% of the KTaO₃.

The reaction for the dissolution of KTaO₃ is believed to occur by thefollowing mechanism:

    2KTaO.sub.3 +14HF(aq)→H.sub.2 TaF.sub.7 (aq)+2KOH(aq)+4H.sub.2 O

Some of the potassium ions may react with the aqueous H₂ TaF₇ in thetantalum-containing solution to form a potassium fluorotantalate, K₂TaF₇, precipitate. However, the precipitated K₂ TaF₇ can easily bedissolved in dilute HF and H₂ SO₄. Thus, the conversion of the technicalgrade Ta₂ O₅ to KTaO₃ followed by dissolution in HF media yields atantalum-containing solution which is appropriate for use inconventional solvent extraction or ion exchange purification methods.

In addition, highly pure capacitor grade K₂ TaF₇ can be directlyprecipitated from the tantalum-containing solution by adding a watersoluble potassium compound such as KCl. The reaction proceeds asfollows:

    H.sub.2 TaF.sub.7 (aq)+2KCl→K.sub.2 TaF.sub.7 (s)↓+2HCl

In a preferred method, the amount of the water soluble potassiumcompound used in the reaction is approximately twice that required bystoichiometry. Other acceptable water soluble potassium compounds whichmay be used to form the K₂ TaF₇ precipitate include K₂ CO₃, KOH, KF, K₂C₂ O₄ and K₂ O.

EXAMPLE 5

Highly pure potassium fluorotantalate was made by forming a mixture of132 g of technical grade Ta₂ O₅ with 45 g of K₂ CO₃. The mixture washeated in an alumina crucible at 750° C. for 5-10 hours to form KTaO₃.After washing the KTaO₃ with water and drying at 100° C., 125 g of thedried KTaO₃ was slurried with 125 ml of concentrated H₂ SO₄ in a plasticbeaker to which was then added 225 ml of concentrated HF. An exothermicreaction occurred converting all of the KTaO₃ to K₂ TaF₇ /H₂ TaF₇. Waterwas added to bring the solution volume to 4 liters. The solution wasseparated from the K₂ TaF₇ precipitate and 100 g of KCl was added to thesolution to complete the precipitation of K₂ TaF₇ which was thenseparated from the solution. Both precipitates were mixed together andwashed with two 11 volumes of 0.5% HF (water washing may also be used).The washed mixture was dried at 100° C. and characterized by x-raydiffraction (XRD) and GD-MS. The XRD results confirmed that theprecipitates were K₂ TaF₇. Table 1 contrasts the purity of theprecipitated K₂ TaF₇ with that of the technical grade Ta₂ I₅. Theseresults show that the niobium and transition metal impurities present inthe technical grade Ta₂ O₅ have been greatly reduced resulting in ahighly pure K₂ TaF₇ which is acceptable for use in the manufacture oftantalum capacitors.

It can also been seen from Table 1 that the purity of potassiumfluorotantalate prepared via dissolution of KTaO₃ (KTaO₃ /H₂ SO₄ /HF)was significantly better than that prepared via direct dissolution ofTa₂ O₅ (Ta₂ O₅ /H₂ SO₄ /HF). The addition of H₂ SO₄ before thedissolution of KTaO₃ in HF promotes the removal of Nb and othertransition metal impurities during the selective crystallization of K₂TaF₇. As can be seen in Table 1, a purer K₂ TaF₇ is obtained by addingH₂ SO₄ to the KTaO₃ prior to dissolution in HF (KTaO₃ /H₂ SO₄ /HF) thandirect dissolution of the KTaO₃ in HF (KTaO₃ /HF).

                  TABLE 1                                                         ______________________________________                                               Concentration, ppm                                                                      K.sub.2 TaF.sub.7 by                                                                     K.sub.2 TaF.sub.7 by                                                                  K.sub.2 TaF.sub.7 by                                       KTaO.sub.3 /                                                                             KTaO.sub.3 /                                                                          TaO.sub.5 /                               Elements Ta.sub.2 O.sub.5                                                                      H.sub.2 SO.sub.4 /HF                                                                     HF only H.sub.2 SO.sub.4 /HF                      ______________________________________                                        Chromium 15      <1         1       1                                         Copper   21       2         2       <1                                        Iron     64       1         8       5.5                                       Lead      2      <1         1       1                                         Manganese                                                                               2      <1         <1      <1                                        Nickel    9      <1         <1      6                                         Niobium  250      4         20      13                                        Tin       2      <1         <1      2                                         Titanium <1      <1         5       2                                         Tungsten 140      2         13      43                                        Zinc      4      <1         2       8                                         ______________________________________                                    

Potassium fluorotantalate (K₂ TaF₇) obtained by the present method isnot only pure but can be converted directly into hydrated ammoniumtantalum oxide without prior dissolution in HF. Because the K₂ TaF₇ isformed instantaneously from a highly supersaturated (supersaturated withrespect to K₂ TaF₇) solution, it has a higher specific surface area thanK₂ TaF₇ prepared by slow crystallization. The K₂ TaF₇ prepared by themethod of this invention had an average surface area of about 0.62 m² /gwhereas samples of commercially available K₂ TaF₇ averaged 0.12±0.5m2/g. Due to its higher surface area, the precipitated K₂ TaF₇ can bedirectly converted into hydrated ammonium tantalum oxide using ammoniumhydroxide. This is a significant improvement over existing technologywhich requires complete dissolution of K₂ TaF₇ in HF before conversionto hydrated ammonium tantalum oxide by reacting with ammonium hydroxide.

The FIGURE illustrates the various pathways by which purifiedtantalum-containing compounds can be obtained after the tantalumpentoxide dissolution step. The center pathway is a preferred wetchemical process for economically producing pure electronic gradetantalum pentoxide. This method does not require the large volumes oforganic solvents used in solvent extraction methods or the relativelyexpensive resin and equipment needed for ion exchange.

In this preferred method, KCl is used to precipitate K₂ TaF₇ from thetantalum containing solution produced in the dissolution step. The K₂TaF₇ precipitate is separated from the liquid by filtration and washedwith water. The wet precipitate is then suspended in water and,optionally, a chelating agent such as ethylenediaminetetraacetic acid(EDTA) is added to sequester transition metals such as iron and copper.After mixing, the K₂ TaF₇ is directly converted into hydrated ammoniumtantalum oxide, (NH₄)₂ Ta₂ O₆ ·H₂ O, by hydrolyzing the suspension with14.5-15M ammonium hydroxide which is added while continuously stirringthe suspension. After 2-3 hours, the (NH₄)₂ Ta₂ O₆ ·H₂ O precipitate isfiltered, washed to neutral pH with water and dried at 110° C. The(NH₄)₂ Ta₂ O₆ ·H₂ O can then be converted into tantalum pentoxide bycalcining at between about 900° to about 1000° C.

Purified tantalum pentoxide was made by forming a mixture of 132 g oftechnical grade tantalum pentoxide, containing approximately 600 ppm ofniobium and other transition metal impurities, and 45 g of potassiumcarbonate. The mixture was heated in an alumina crucible at 750° C. to850° C. for 5 to 10 hours to form potassium tantalate which was washedwith water and dried at 110° C. A slurry of 125 g of the potassiumtantalate and 125 ml of concentrated sulfuric acid was formed in a 4 lplastic beaker to which 225 ml of concentrated HF was then added. Anexothermic reaction occurred and all of the potassium tantalate wasconverted into potassium/hydrogen fluorotantalate. Water was added tomake a total volume of 4 l. A potassium fluorotantalate precipitate wasformed by adding 100 g of KCl to the solution. After the crystals ofpotassium fluorotantalate had settled, the supernate was removed bydecanting, leaving about 1 l of slurry. One liter of water was added tothe slurry and the K₂ TaF₇ precipitate was separated by filtration,washed with 1.5 l of water and resuspended into 6 l of water. Thesuspension was shaken well and 1 g of EDTA was added. A whiteprecipitate was then formed by adding 800 ml of 14.5-15M ammoniumhydroxide while continuously stirring the suspension. The whiteprecipitate was left in the mother liquor for 2 hours to 3 days. Theprecipitate was filtered, washed to neutral pH, and dried at 110° C. toobtain hydrated ammonium tantalum oxide. Purified tantalum pentoxide wasobtained by calcining the hydrated ammonium tantalum pentoxide atbetween 900° C. to 1000° C. in a silica boat. Samples from each stagewere characterized by x-ray diffraction (XRD), glow discharge massspectrometry (GDMS), and infra-red (IR) spectroscopy methods.

The purity of the tantalum pentoxide produced by the preferred wetchemical method is shown in Tables 3 and 4. In Table 3, the purity ofthe technical grade tantalum pentoxide used as the raw material iscompared with the purity of the resultant purified tantalum pentoxidefor three separate batches. The purity of tantalum pentoxide preparedfrom two different technical grade tantalum pentoxide sources is givenin Table 4.

                  TABLE 3                                                         ______________________________________                                                Concentration, ppm                                                              Standard                                                                      Technical Purified  Purified                                                                             Purified                                           Grade     Ta.sub.2 O.sub.5                                                                        Ta.sub.2 O.sub.5                                                                     Ta.sub.2 O.sub.5                         Elements  Ta.sub.2 O.sub.5                                                                        Batch #1  Batch #2                                                                             Batch #3                                 ______________________________________                                        Bismuth   <1        <1        <1     <1                                       Chromium  15        <1        1      <1                                       Cobalt    <1        <1        <1     <1                                       Copper    21        3         2      3                                        Iron      64        3         2      2                                        Lead      2         <1        <1     <1                                       Manganese 2         <1        <1     <1                                       Nickel    9         <1        <1     <1                                       Niobium   250       10        9      9                                        Thorium   <1        <1        <1     <1                                       Tin       2         <1        <1     <1                                       Titanium  <1        <1        <1     <1                                       Tungsten  140       9         7      5                                        Vanadium  <1        <1        <1     <1                                       Zinc      4         <1        <1     <1                                       Zirconium <1        <1        <1     <1                                       ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                               Concentration, ppm                                                                        Purified-         Purified-                                Element  Ta.sub.2 O.sub.5 (A)                                                                    Ta.sub.2 O.sub.5 (A)                                                                    Ta.sub.2 O.sub.5 (B)                                                                  Ta.sub.2 O.sub.5 (B)                     ______________________________________                                        Titanium 5         1         1       <1                                       Vanadium <1        <1        <1      <1                                       Chromium 88        10        33      2                                        Manganese                                                                              7         <1        6       <1                                       Iron     240       12        81      3                                        Cobalt   <1        <1        8       <1                                       Nickel   60        6         18      <1                                       Copper   18        <1        2000    4                                        Zinc     13        1         --      4                                        Arsenic  <1        <1        <1      <1                                       Zirconium                                                                              2         <1        <1      <1                                       Niobium  130       2         180     7                                        Molybdenum                                                                             3         1         3       2                                        Tin      37        1         1       <1                                       Tungsten 4         4         8700    13                                       Lead     2         <1        <1      <1                                       Bismuth  <1        <1        <1      <1                                       Thorium  <1        <1        <1      <1                                       ______________________________________                                    

Thus, a pure electronic grade tantalum pentoxide can be obtained withoutusing solvent extraction or ion exchange purification methods.

While there has been shown and described what are at the presentconsidered the preferred embodiments of the invention, it will beobvious to those skilled in the art that various changes andmodifications may be made therein without departing from the scope ofthe invention as defined by the appended claims.

We claim:
 1. A method for the dissolution and purification of tantalumpentoxide comprising reacting tantalum pentoxide and apotassium-containing compound to form potassium tantalate, dissolvingthe potassium tantalate in a hydrofluoric acid medium to form a solutioncontaining tantalum values and impurities and separating the tantalumvalues from the impurities.
 2. The method of claim 1 wherein thepotassium-containing compound is potassium carbonate.
 3. The method ofclaim 2 wherein stoichiometric amounts of tantalum pentoxide andpotassium carbonate are reacted at a temperature between about 550° C.to about 1300° C. for a time of about 3 hours to about 20 hours.
 4. Themethod of claim 3 wherein the temperature is about 750° C. to about 850°C. and the time is about 5 hours to about 10 hours.
 5. The method ofclaim 1 wherein the potassium tantalate is slurried with sulfuric acidbefore being dissolved in the hydrofluoric acid medium.
 6. The method ofclaim 1 wherein the hydrofluoric acid medium is concentratedhydrofluoric acid.
 7. The method of claim 1 wherein the tantalum valuesand the impurities are separated by ion exchange.
 8. The method of claim1 wherein the tantalum values and the impurities are separated bysolvent extraction.
 9. The method of claim 1 wherein the tantalum valuesare separated from the impurities by adding a water soluble potassiumcompound to the solution to form a K₂ TaF₇ precipitate.
 10. The methodof claim 9 wherein the K₂ TaF₇ precipitate is washed with dilute HF orwater and dried to form capacitor grade K₂ TaF₇.
 11. The method of claim9 further comprising suspending the K₂ TaF₇ precipitate in an aqueoussolution optionally containing a chelating agent, and adding ammoniumhydroxide to form an ammonium tantalum oxide precipitate.
 12. The methodof claim 11 further comprising filtering, washing, drying and calciningthe ammonium tantalum oxide precipitate at a temperature between about900° C. to about 1000° C. to obtain a purified tantalum pentoxide. 13.The method of claim 9 wherein the water soluble potassium compound isselected from the group consisting of K₂ CO₃, KOH, KF, KCl, K₂ C₂ O₄ andK₂ O.
 14. A method for the dissolution and purification of tantalumpentoxide comprising reacting stoichiometric amounts of an impuretantalum pentoxide and potassium carbonate to form potassium tantalate,optionally slurrying the potassium tantalate with sulfuric acid,dissolving the potassium tantalate in an hydrofluoric acid medium toform a solution containing tantalum values and impurities, precipitatingpotassium fluorotantalate by adding a water soluble potassium compound,filtering the solution to obtain a wet precipitate, suspending the wetprecipitate in water, optionally adding an effective amount of achelating agent, precipitating (NH₄)₂ Ta₂ O₆ ·H₂ O by adding ammoniumhydroxide and calcining the (NH₄)₂ Ta₂ O₆ ·H₂ O to obtain a tantalumpentoxide having low amounts of niobium and transition metals.
 15. Themethod of claim 14 wherein the water soluble potassium compound is KCl.16. The method of claim 14 wherein the chelating agent is EDTA.